Purified peritoneal macrophages from allograft-bearing mice were shown by 51Cr release cytotoxicity assays to be specifically cytolytic for the appropriate cells. Maximum reactivity was detected approximately 12 days after the S.C. inoculation of allogeneic leukemic cells. I.P. treatment of allograft-bearing mice with pyran acted synergistically to potentiate specific macrophage reactivity. The concentration of different prostaglandins (PGs) was determined in the culture of medium of growing BALB/3T3 and virus transformed SV3T3 mouse fibroblasts. The data clearly demonstrates that for cell PGs measured, higher levels were detected in the medium of the SV-40 transformed cells. Of the three series of PGs studied, a 14-fold increase in PGE was detected in the cultures of the SV-40 transformed cells. Supernatants obtained from mouse fibrosarcoma cultures 48 hours after the addition of fresh medium contained a dialyzable material which inhibited the proliferation of syngeneic lymphoma cells in vitro, as measured by 3H-thymidine incorporation. Several lines of evidence indicate that the supernatant inhibitory material is probably PGE. The migration characteristics of the inhibitory substance is the same as authentic PGE as detected by TLC, and indomethacin treatment of fibrosarcoma cultures reduced the amount of supernatant inhibitory substance present. BIBLIOGRAPHIC REFERENCES: Schultz, R.M., Papamatheakis, J.D., Stylos, W.A., and Chirigos, M.A.: Augmentation of specific macrophage-mediated cytotoxicity: correlation with agents which enhance antitumor resistance. Cellular Immunol. 25: 309-316, 1976. Sonis, S.T., Stelos, P., Stylos, W.A., and Wilson, R.E.: Inhibition of lymphoma cell proliferation by supernatant from fibrosarcoma cultures: preliminary evidence that the inhibitory material is prostaglandin E. Prostaglandins. 13: 87-96, 1977.